1. Field of Invention
This invention relates generally to internal combustion engine cooling systems and more particularly to liquid cooled automotive vehicle engines which employ liquid to air heat exchangers, referred to as "radiators", and associated fans for forcing a flow of cooling air into heat exchange relationship with the radiators.
The efficiency of a fan is dependent in part upon the quantity of air which moves past the blade tips from the discharge side to the suction side of the fan. This air movement, which is referred to as "recirculation" can be reduced significantly, but not eliminated entirely, if the fan blade is surrounded by a shroud. The quantity of recirculated air in a shrouded fan is dependent in part upon the radial clearance between the shroud and the fan blade tip portions. When the clearance is reduced the fan efficiency tends to be increased.
Automotive vehicles employing liquid cooled engines are frequently provided with shrouded fans for forcing flows of cooling air across the coolant system radiators. In farm tractors or highway trucks the fan and surrounding shroud are frequently separately mounted. For example, in many such vehicles the fan is carried by the engine while the shroud is attached to the radiator. When such a vehicle is operated over rough terrain or rough roadways, the fan and shroud can experience relatively significant motion relative to each other, necessitating provision of a substantial clearance between the fan blade tips and the shroud to prevent the fan blades from striking the shroud at the extremes of the relative motion.
2. Prior Art
A number of devices have been proposed to reduce the clearance between fan blades and a surrounding shroud while still allowing for motion of the fan radially and axially relative to the shroud without substantial risk of resultant collision damage. These devices have either limited the relative radial and axial motion between the fans and the shrouds or provided clearances radially outwardly from the blades which has resulted in less than optimum fan efficiency due to air recirculation through the clearance.
One proposed solution provided a flexible sealing assembly received by a recess in the fan shroud, and a seal engaging assembly attached to the fan blades and riding, as if in a track, in the recess. This approach enabled only relatively limited radial and axial relative motion between the fan and shroud.
In another proposed solution an inflatable tube was mounted in the clearance between the fan blade tips and shroud. The tube was inflated to reduce clearance between the blade tips and the shroud when the fan was required to direct a maximized flow of cooling air across the radiator, and was deflated when the engine cooling system load was small and did not require a high degree of fan efficiency. When relative motion occurred between the fan and the shroud, collisions between the fan blades and the inflatable tube were frequent and precipitated early tube failures.
In still another proposal a resilient membrane, slotted for increased resiliency, was attached circumferentially about the shroud in the plane of the fan and was spaced radially from the fan blades. When large amounts of air were rammed through the fan by virtue of vehicle forward motion, the resiliently flexible membrane was deflected away from the fan blades by the ram air flow. This proposal tended to reduce the possibility of fan blade-shroud collisions at high vehicle speeds but was not as effective at low vehicle speeds.
Another proposal involved attaching an abradable foam strip about the inner shroud surface occupying most of the clearance between the shroud and fan. The fan abraded the foam strip during periods of relative motion between the fan and the shroud until a sufficiently large clearances was formed between the fan blades and the shroud. This abrasion process resulted in a clearance between the fan blades and the foam strip and as a result achieved only a partial reduction in recirculation of air around the fan.